Nothing. This is a iinguistic issue, which I am not professional.
However, I know some basic Khmer words which are identical to Vietnamese:
numbers from 1 to 5 (for example: boyn = bốn; myot - một)
ka = cá
In addition, I doubt if a Spanish and an Italian can understand each other without studying. My Russian mate (20 years ago) said that Russian need to study around 6 months to understand other Slavic languages (similar to an adult Vietnamese learn spoken Chinese)
And I know for sure that Romanian and Italian cannot understand each other (both are Latin languages).
And Bengali (spoken in Bangladesh) and Norse (spoken in Norway) both belong to Indo-European language family (with many similar basic words, may be even more than the similarity between Vietnamese and Khmer) but I doubt a Bengali can understand a Norwegian.